Currently there is no easy-to-use, inexpensive method for measuring the concentrations of many stable isotopes. As enriched stable isotopes become more available for many important agricultural, medical and physical studies, the lack of such a method will prevent the more general usage of these valuable chemical tracers.
Inductively coupled plasma (ICP) emission spectroscopy is an established ultrasensitive analytical method which has been used to measure trace metals at the sub-parts-per-billion level. A typical commercially available ICP emission spectrometer consists of three major components, viz., a microcomputer and sample introduction system, the plasma source system, and the emission spectrometer and detection system. Trace metal analysis is achieved by quantitatively measuring atomic optical emission from the plasma.
Examples of prior art ICP devices include those disclosed in U.S. Pat. Nos. 3,467,471 (Greenfield et al); 4,293,220 (Denton et al); 4,266,113 (Denton et al); and 3,958,883 (Turner). Examples of plasma torches which are driven by microwave radiation include those disclosed in U.S. Pat. Nos. 4,225,235 (Anderson et al) and 3,843,257 (Wooten). Other patents of possible interest include 3,514,604 (Trojean) and 3,635,561 (Bordonali et al). Briefly considering some of these patents, the Greenfield et al patent discloses an inductively coupled plasma torch which is used as a spectroscopic source and which employs an "insulating fluid" that surrounds the plasma. The "insulating fluid" is apparently a coolant gas, although, as discussed below, mention is also made of the use of water for this purpose. Reference is made elsewhere in the patent to water cooling of the induction coil. The requirements of this torch include high gas flow and high power. The Denton et al patents disclose ICP torches similar to that of the Greenfield et al patent and which likewise are characterized by high r.f. power demands, high gas flow rate and a water cooled r.f. induction coil. The torch is designed to minimize molecular band emission. The torch of the Turner patent is also similar, the torch operating at high pressure and being designed to maximize atomic emission and minimize molecular emission. As noted above, the Anderson et al and Wooten patents concern torches which are driven by microwave radiation and both are concerned with a design and operation of the torch which minimizes molecular emission. The significance of the teachings of the prior art and the differences between the prior art and the present invention will be discussed below.